1. Technical Field
The present invention relates to a sealing glass composition, in particular, one which is able to seal a panel and funnel of color TV at low temperature for a short time by adding Garnet(Fe.sub.3 Al.sub.2 Si.sub.3 O.sub.12) as a nucleating agent and filler component to control thermal expansion coefficient.
2. Description of Related Prior Art
As disclosed in U.S. Pat. No. 2,889,952, a crystalline sealing glass composition used for sealing panel and funnel of color TV is a PbO--ZnO--B.sub.2 O.sub.3 -based glass, having a sealing temperature of 440.degree. C. and a sealing time of about 1 hour. However, such a high sealing temperature and long sealing time decreases the productivity of manufacturing a color TV Braun tube.
Accordingly, in order to increase its productivity in manufacturing CRT for color TV, development and utilization of a sealing glass composition capable of lowering sealing temperature and reducing sealing time has been required.
Until now the development thereof has been carried out is two ways, that is, one is lowering sealing temperature and the other is reducing sealing time.
First, to develop sealing glass compositions which have a low melting point, glass softening temperature, and which are able to reduce sealing temperature are disclosed in U.S. Pat. Nos. 4,097,296 and 4,246,034 with PbO--ZnO--B.sub.2 O.sub.3 -based glass containing fluorine.
Generally, PbO--ZnO--B.sub.2 O.sub.3 -based glass compositions containing fluorine have to be used with low expansion filler because its thermal expansion coefficient is too high and its mechanical strength is low. Generally, a mixture of mother glass and one or more compounds selected from natural zircon(ZrSiO.sub.4), lead titanate (PbTiO.sub.3), cordierite, willemite, .beta.-eucryptite and .beta.-spodumene are used as filler.
The content of the filler is different from the composition of mother glass powder. U.S. Pat. No. 4,097,296 discloses a glass composition with 2.about.6 wt % of natural zircon. And U.S. Pat. No. 4,246,034 discloses a glass composition with 4.about.10 wt % of cordierite.
Sealing temperature of the glass composition is lowered by 20.degree. C. compared with the conventional composition, although the sealing keeping time(45minutes to 1 hour) of the glass composition is longer by 15.about.30 minutes than conventional products.
In order to improve the productivity of a color TV Braun tube, reducing sealing time is more important than lowering sealing temperature. Accordingly, a sealing glass composition with a low sealing temperature didn't lead to improved productivity in manufacturing color TV Braun tube.
Decreased flowability by adding large amounts of filler into the glass composition adversely affects sealing.
Addition of alkaline oxidant, to the glass composition is desirable in that sealing temperature is lowered and flowability is good, but such additions cause a problem with sealing of the panel and funnel of a color TV requiring high insulation because alkaline ion in glass has high electric mobility.
The second direction taken in the development of a sealing glass composition is reducing sealing time.
U.S. Pat. No. 3,947,279 discloses composition which contains precrystallized PbO--ZnO--B.sub.2 O.sub.3 -based glass as a seed and is able to seal a panel and funnel for 28 minutes at a sealing temperature of 445.degree. C. U.S. Pat. No. 4,589,899 discloses composition which contains zinc zirconium silicate as a nucleating agent and is able to seal for 15.about.25 minutes at 440.degree. C.
However, as above noted, use of a nucleating agent reduces crystallization time, but the flowability necessary to effective seal, may be lost because excess crystallization occurs before a flow is sufficiently conducted and glass mechanical strength is decreased due to excess growth of crystal.
Because it has the same sealing temperature or higher than that of the prior art, it has a limitation to improve the productivity of color TV Braun tube.
From the viewpoint of reducing sealing time, sealing is possible even when crystallization is not sufficient. However, cracking on sealing glass may a rise during a vaccum exhaust process around at 350.about.380.degree. C. after sealing a panel and funnel of glass, because thermal expansion coefficient changes rapidly around at glass transition temperature due to excess of non-crystallized part. Although cracking of sealing glass would not be a problem, mechanical strength of sealing glass would be decreased.
If the sealing glass compositions content of natural zircon is increased to reduce sealing time, flowability cannot be ensured by preventing wetting, and the flowability of sealing glass required for sufficient sealing may not occur, because zircon's role as filler is superior to that as nucleating agent and its thermal expansion coefficient is extremly decreased.